Micro-machined thermal infrared (IR) detectors are a well-established technology, and are typically based on thermopiles, bolometers, pyrodetectors or even diodes. These typically include a structure thermally insulated from the substrate (such as a membrane or micro-bridge) which heats up due to incident IR radiation, and this change in temperature is detected using various methods.
A micro-machined infrared (IR) thermopile detector typically comprises a number of thermocouples connected in series with their hot junctions (sensing junctions) embedded within a membrane, or any other thermally isolating structure (e.g. a bridge, a cantilever, etc.), and their cold junctions (reference junctions) located outside the membrane, or any other thermally isolating structure.
Each thermocouple is formed by joining together two dissimilar materials (i.e. thermocouple legs). Additionally an IR thermopile detector comprises two electrical connections (i.e. metallic tracks) connecting the thermopile positive and negative ends to the respective PADs on-chip. As a consequence an IR thermopile detector will be characterised by two outputs and will need a package with at least two leads, or any other means for external connection. In case of an IR thermopile detector array, the number of outputs will be proportional to the number of thermopile detectors forming the array. For the general case of an array comprising N thermopiles, the number of outputs will be 2N.
Having a large number of outputs is not desirable, since it has an effect on:                i) the number of PADs on-chip, resulting in larger occupied area, and thus higher costs;        ii) the number of bonding wires, with related costs and possibly arising reliability issues; and        iii) the number of package leads, or any other means for external connection, with related cost and package size.        
Furthermore, in some applications there is no interest in the absolute output from each single IR detector forming the IR detector array, but there is interest in the differential output (i.e. the difference between outputs from different thermopiles).
A number of designs of IR thermopile detectors have been reported.
For example, in A. De Luca, et al., “Filterless non-dispersive infra-red gas detection: A proof of concept,” in Micro Electro Mechanical Systems (MEMS), 2017 IEEE 30th International Conference on, 2017, pp. 1220-1223, an array of IR detectors comprising two IR thermopile detectors with tailored optical properties is presented and the quantity of interest is the difference between the two thermopiles' output that is correlated to CO2 concentration.
Graf et al. “Review of micromachined thermopiles for infrared detection” Meas. Sci. Technol. 18 (2007) R59-R75 reviews several thermopile based IR detectors which are typically on a membrane.
Typically the entire thermally isolated area is considered as sensing area, but in some cases the sensing area is defined as the portion of the area of the thermally insulating structure comprising an IR absorbing layer. In A. De Luca, et al., “Enhanced spectroscopic gas sensors using in-situ grown carbon nanotubes,” Applied Physics Letters, vol. 106, p. 194101, 2015 the sensing area is the area where carbon nanotubes are grown, and in U.S. Pat. No. 9,214,604B2 by Ali et al. the sensing area is the area where the plasmonic structures are located.
It is also well known how to fabricate arrays of IR detectors. For example, Hirota et al., “120×90 Element thermopile array fabricated with CMOS technology,” Proceedings of SPIE Vol. 4820 (2003) pp. 239-249 describes an array of thermopile IR detectors, where each IR detector pixel is a separate front-side etched membrane.
Sarro et al., “An integrated thermal infrared sensing array,” Sensors and Actuators 14 (1998) pp. 191-201, describes a linear 8-element thermopile array where each IR detector is on a cantilever structure. Jones et al., “MEMS thermal imager with optical readout”, Sensors and actuators A 155 (2009), pp. 47-57, describes a two dimensional array where each detector is on a cantilever.
Foote et al., “High performance micromachined thermopile linear arrays,” SPIE Vol. 3379, 1998, pp. 192-197, describes a linear array with each thermopile IR detector on a micro-bridge.
Calaza et al., “An uncooled infrared focal plane array for low-cost applications fabricated with standard CMOS technology,” Sensors and Actuators A 132 (2006) pp. 129-138, describes a two dimensional IR detector array, where each detector is on a suspended membrane/microbridge structure.
Kanno et al., “Uncooled infrared focal plane array having 128×128 thermopile detector elements,” SPIE Vol. 2269, pp. 450-459 describes a 128×128 IR detector array, where each element is on a suspended membrane/diaphragm. U.S. Pat. No. 7,842,922 describes an IR detector array based on thermopiles, where each element is on membrane.